The present invention relates to equipment and methods for the photolithography of photoresist-coated substrates such as those used for plasma display television screens.
The exposure of small substrates such as those used for plasma display screens is generally performed with both the substrate and the photo tool or mask in a horizontal position by a single exposure from a vertically directed collimated light beam. However, for a number of reasons, such methods do not work well for large panels such as those used for large plasma display television screens. One problem is that for a large substrate, it is difficult to provide a collimated light beam of a sufficient size to permit the single exposure of the substrate at a uniform intensity over its entire surface. Another problem is that when a large mask is held in a horizontal mask frame for exposure of a substrate, the center of the mask tends to sag due to its weight. Similarly, unless it is properly supported, a horizontal substrate will tend to sag. Any such sagging in the mask or substrate makes the accurate photolithography of the substrate virtually impossible.
While a number of manufacturing techniques have been developed in attempts to avoid these problems, such techniques have proven to be difficult and costly to implement. Moreover, current methods do not generally permit the quick and accurate exposure of a large number of large substrates in an automated fashion.
Generally, the methods for exposing large substrates have required either scanning the entire surface of the panel with a moving beam and/or using a fixed beam and moving the mask and substrate so as to expose the substrate""s entire surface by a scanning procedure. These scanning methods include the xe2x80x9cstep and repeatxe2x80x9d method in which specific portions of the panel are sequentially exposed in incremental steps, each step by a timed exposure. Other methods include a continuous scan in which the substrate and light beam are moved with respect to one another in a moving exposure. One such continuous scan method moves the mask and substrate over a serpentine path to expose the substrate to a fixed collimated light beam in a single exposure until the entire substrate has been exposed.
Both step and repeat and continuous scan methods of exposure often lead to stitching errors, that is, errors where discrete lines of overexposure or underexposure are left in the substrate where adjacent paths of the collimated light beam do not perfectly mesh with one another. To the extent stitching errors can be reduced, these basic methods are nonetheless time consuming and often difficult to consistently repeat for a large batch of substrates. Problems with repeatability lead to the frequent rejection of substrates for failing to meet minimum quality control standards.
With regard to the problem of mask sagging when a large mask is held in a horizontal position for exposure of a substrate using a vertical light beam, this problem has largely been overcome through the use of a generally horizontal beam of exposure light that is directed through a mask and substrate held in a generally vertical orientation. However, when the mask and substrate are held perfectly vertical, they tend to be unstable and difficult to hold motionless during the exposure. The stability of the mask and substrate can be improved while reducing sag by holding the mask and substrate in planes that are slightly out of vertical. Nevertheless, while sag problems can generally be avoided by these methods, the handling of large, vertically oriented masks and substrates has proven to be difficult. These problems in handling make conventional exposure methods for large substrates costly, time consuming and unreliable.
Yet another difficulty with existing methods for exposing large substrates has been that such exposures must generally be done in a clean room so as to avoid contaminating either the mask or the photoresist coating on the substrate with dust or dirt. Because the equipment associated with the exposure of large substrates is very large, it is often difficult and costly to fit such equipment within a clean room. Moreover, once the equipment is installed in a clean room, any routine maintenance can be difficult and costly due to the steps that must be taken in maintaining the clean room atmosphere.
An improved apparatus and method for exposing large photoresist-coated substrates quickly, reliably and at low cost is desired.
According to the present invention, an improved apparatus and an improved method for exposing large photoresist-coated substrates are provided. The apparatus includes two key components: a light scanning assembly, and a mask and substrate handling assembly. One important advantage to the apparatus is that the two key components need not be physically located in the same room. Therefore, only the mask and substrate handling equipment need to be located in a clean room. The light source can be located in an adjacent room. By locating the light source outside the clean room the ability to perform routine maintenance on the light scanning assembly is greatly simplified.
According to the invention, the mask and substrate handling assembly includes a frame for holding the mask in a substantially vertical position. The mask is held in place on as vacuum chuck. It is generally desired to hold the mask at an angle slightly out of vertical, preferably at an angle up to about 5 degrees from vertical, and more about 2 degrees from vertical, in order to effectively eliminate sag while improving the stability of the mask. A 2 degree angle is achieved by mounting the frame perpendicularly to a ramp with a 2 degree slope from horizontal. The ramp is mounted on a base that is preferably anchored to a block of a heavy material such as granite so as to damp out any vibrations from the building in which the equipment is located. The frame is mounted to the ramp with slide bearings and a positioning table driven by a stepper motor is provided to enable the frame to be moved up and down the ramp.
A tiltable substrate platen, also mounted on the base, is provided for receiving the photoresist-coated substrate in a generally horizontal position. The platen includes pneumatically-driven snubbing pins which cooperate with banking pins to center the substrate on the platen. A vacuum chuck is provided on the platen to hold the centered substrate firmly in place. The side of the platen proximate the frame is hinged to permit the substrate to be tilted into a position parallel to the mask. The platen is tilted by a servo-driven jack screw assisted by a pair of pneumatic lift assists. Preferably, the mask and substrate are oriented with respect to one another so that the preferred 2 degree tilt of the mask is in the direction of the substrate.
The mask and substrate handling assembly further includes equipment for automatically moving a substrate into position on the platen. A horizontal in-feed conveyor is provided adjacent the platen with a plurality of drive rollers to feed a substrate to the platen. In order to assist the platen in receiving the substrate from the in-feed conveyor, the platen includes a plurality of retractable wheels which can be extended up from the surface of the platen and driven by a motor to assist in maneuvering the substrate to the proper position on the platen. The wheels retract to lower the substrate so that it can be held in place by the vacuum chuck. Similar to the in-feed conveyor, an out-feed conveyor is provided adjacent the platen to withdraw the exposed substrate from the platen for further processing. Like the in-feed conveyor, the out-feed conveyor includes a plurality of drive rollers for manipulating the exposed substrate.
The light scanning assembly includes collimated light beam projection equipment mounted on a shuttle. The collimated light beam projection equipment includes a lamphouse which directs a beam of ultra violet light to a reflecting mirror which, in turn, directs the light beam to a collimating mirror. The equipment is mounted on the shuttle so that the collimated light beam from the collimating mirror will be directed perpendicular to the mask. In the preferred embodiment, the collimated light beam is directed at a 2 degree angle from horizontal. The collimated light beam provided by this equipment is of a sufficient height to permit the entire substrate to be exposed in a single and continuous horizontal pass.
In order to permit an accurate horizontal scan of the substrate, the shuttle is mounted on a plurality of horizontal rails. The shuttle is moved along the rails by a screw which is driven by a servo motor.
In the preferred embodiment the apparatus is automated through the use of a microprocessor. In order to expose a batch of large photoresist-coated substrate material, a first sheet of substrate is driven by the rollers of the in-feed conveyor toward the platen. The retractable wheels of the platen are extended to receive the substrate which is then moved by the wheels toward the center of the platen. Once positioned, the wheels are retracted and the snubbing pins are engaged to center the substrate on the platen against the banking pins. Once centered, the vacuum chuck is engaged to hold the substrate to the platen. The substrate is then tilted to a position parallel to the mask using the servo-driven jack screw and pneumatic lift assists.
Once the substrate has been tilted into position, the mask is moved into close proximity with the substrate by moving the frame down the ramp using the positioning table. The precise spacing between the mask and substrate can be achieved through a number of different ways. In one embodiment, gap sensors can be used to determine the spacing between the mask and substrate and control the movement of the mask toward the substrate. However, in the preferred embodiment, a set of shims are provided on the mask which abut against the substrate to provide the proper gap between the mask and substrate. Once the substrate and mask are in proximity to one another, the scanning exposure can begin. The speed at which the shuttle is driven along the track can be used to control the exposure level and can also be used to compensate for output losses from the light source which commonly occur with the aging of the UV lamp elements.
Once an exposure is complete, the frame is pushed up the ramp to separate the mask from the substrate. The platen is then tilted back to its horizontal position and the vacuum chuck is disengaged to release the substrate. The retractable wheels are then extended and used to drive the exposed substrate to the out-feed rollers which withdraw the substrate from the platen for further processing. Simultaneously, the in-feed rollers are engaged to feed a second substrate to the platen. The process is repeated with one difference being that the second substrate is exposed with a scan in a direction opposite to that of the first substrate. By scanning the odd substrates in a first direction, for example, from left to right, and the even substrates in a second direction, right to left, a large number of substrates can more rapidly be exposed while reducing the travel of the shuttle.
In the preferred embodiment, the frame farther includes alignment equipment which is especially useful with substrates requiring multiple exposures. For such multiple exposures, it is critical that the mask pattern for a particular exposure be precisely aligned with the patterns of any previous exposures. The alignment equipment permits the manipulation of the mask within its plane in up and down, side to side and rotational directions. However, this manipulation is all maintained in the same plane so that the mask and substrate will remain parallel to one another during exposure. Such manipulation is accomplished by using a mask frame made up of a front frame that is adjustably mounted to a back frame. In such an embodiment, the back frame is slidably mounted to the ramp and the front frame is mounted to the back frame with a plurality of positioning tables driven by stepper motors to permit the desired movement of the front frame with respect to the back frame. Both the movement of the front frame up and down in the plane of the mask as well as the rotational movement of the front frame in the plane of the mask are accomplished by a pair of generally vertical positioning tables mounted on the right and left sides of the front frame. Movement of the front frame from side to side is accomplished by a horizontal positioning table. Stacked bearing assemblies including slide and roller bearings help provide rigidity in coupling the front frame to the back frame while permitting the desired movement between the two.
The alignment equipment further includes a plurality of optical sensors which are used to measure the alignment between a mask and substrate. When a substrate is to be subjected to multiple exposures, the first exposure is performed with a mask designed to provide a target at each of the four corners of the substrate. Similar targets are provided on subsequent masks. While the alignment between the mask and substrate is generally not critical for the first exposure, when the substrate is to be exposed for second and perhaps further exposures, the targets on the substrate need to be precisely aligned with the targets on the subsequent masks to ensure that the finished pattern for the substrate is accurate.
Preferably, four optical sensors are provided at the four corners of the frame. The sensors are mounted on retractable arms so as they can be retracted out of the way during scanning. Once the mask frame has been slid down the ramp to place the mask and substrate in proximity with one another, each optical sensor is extended on its respective arm so that it can take a reading to measure the alignment between the targets of the mask and substrate. Signals are sent from the optical sensors to the microprocessor which determines the extent of misalignment and calculates a desired manipulation of the front frame with respect to the back frame to correct any error in alignment. The back frame is then slid up the ramp to move the mask and substrate away from one another. The positioning tables mounting the front frame to the back frame are then manipulated according to signals generated by the microprocessor. The back frame is then slid down the ramp again to place the mask and substrate back into proximity with one another and the optical sensors are again engaged to send new signals to the microprocessor. At this point, the microprocessor will either confirm that the mask and substrate have been aligned within a given tolerance, or calculate new manipulations for the positioning tables. This procedure is repeated until an acceptable tolerance is achieved. The mask and substrate should generally be disengaged from one another between adjustments of the positioning tables so as to avoid damage to the shims by any.abrasion between the substrate and the shims. Such damage could adversely affect the accuracy of later exposures.
Various additional sensors and feedback devices are also preferably used along with microprocessor and the other devices mentioned above to keep the system operating automatically so that a large number of large area substrates can be processed quickly and accurately. The present invention dramatically decreases the amount of time taken to expose a batch of large area substrates yet reliably provides accurate exposures of the substrates, even where multiple exposure of a given substrate is required.